This invention pertains to differential torque limiting devices for mechanical power transmission, and more particularly, to such devices having a single input with two outputs driving two independent actuators which in turn drive a single control surface or device which will tolerate only limited assymmetric loads.
Typically, a single moveable auxiliary airfoil, e.g. a horizontal stabilizer may be actuated by a single actuator although it may employ a torque tube and multiple linkages. However, a single actuator restricts provisions for suitable redundancy in the system in the event of failure of the actuator, its structural support, or its connection to the control surface. Particularly acute is a horizontal stabilizer drive system wherein the entire stabilizer is rotated to provide trim. In the event of an actuator load path failure, the entire stabilizer would be free to rotate about its hinge line and the elevators could not adequately compensate. Dual actuators, either of which will support the stabilizer, but not necessarily drive it alone, provide adequate redundancy. Acme jackscrews, which are inherently no-back devices, meet these requirements. However, employment of dual actuators necessitates synchonization of the actuators. Also the airfoil section rotated must be structurally capable of withstanding a failed or jammed single actuator where one actuator may experience the entire force associated with stall torque of the drive means. Alternately, provisions must be made to avoid this load condition.
Driving the two actuators through a conventional differential gear arrangement wherein the alternate drive outputs are the two driven bevel gears is an alternative well known in the art. In this mode, both output gears are driven only if they are loaded essentially equally. If unequally loaded, the differential pinion will walk about the more heavily loaded first driven bevel gear driving the remaining second driven bevel gear at twice normal speed. The output torque of the two driven bevel gears will be essentially equal. However, since the two actuators must stay in sync with each other, the more lightly loaded advancing screw will jam and become the heavier loaded at which time the second driven gear becomes fixed and the first driven gear rotates. This alternate cycling may continue until both outputs are essentially jammed, at which time the output torque is equally divided. Stall input torque essentially, would be shared equally between the two actuators. Alternately, if the load is lost on one driven bevel gear the differential pinion gear will rotate about the face of the second driven bevel gear (which is still loaded), simply driving the free driven bevel at twice normal speed and applying only the torque to overcome friction.
The obvious problem with this alternative is that the actuator screws may not be maintained in synchronization.
An enhanced embodiment, not hereby conceded to be prior art, is to fasten the differential pinion gear located between the two driven bevel gears to its shaft by a shear pin. This provides a dual (parallel) output mechanical drive with an asymmetrical output load limited to the shear force of the pin holding the pin gear. Once the pinion shears the apparatus is the conventional differential gear drive discussed above.
One undesirable feature with a single shear pin device is that the shear pin must be of adequate size to prevent shearing under maximum load conditions. Consequently at usual (low) operating loads the drive is not sensitive to common faults of a jackscrew actuator--e.g. lack of lubrication or dirt causing high friction and reduced output capability. High load capability from both actuators is required for dive recovery. Since dive recovery is rarely required, it is desirable to have a positive indication of high load capability during usual (low) load operation. Differential load between the two actuator drives at low loads is a good indication of actuator drive capability. The present invention provides a positive indication of actuator capability under usual operation by combining a shear pin and a brake. The shear pin will shear when the actuators have unmatched capability (high differential loading) such as when one actuator is not lubricated or dirty. Opportunity for visual observation of the shear pin is provided for the maintenance crew. With the pin sheared, trimming operation may be continued since the brake provides the differential lock for usual loading conditions.